1. Field of the Invention
The present invention relates to a recording apparatus for image recording.
2. Description of the Prior Art
Conventional recording apparatus such as copier or laser beam printer have generally been equipped with plural indicators, for example an indicator for the absence of recording sheets, an indicator for sheet jamming in addition to a 7-segment indicator unit for recording the number of image recordings. The presence of such plural indicators not only complicates the inspecting work of the operator but also requires a larger space for installation, thus leading to a complicated apparatus.
Also, in the field of recording apparatus, there have been proposed various jam detectors for detecting abnormal transportation of recording sheets, mostly relying on electric timers. Such detectors often suffer from changes in working time due to deterioration of performance in electric components after prolonged use, and sometimes require the use of particular electric timers.
Digital computers employed for controlling the recording apparatus are designed to be automatically reset and start function from a determined step according to predetermined program at the start of power supply. Such computers may perform erroneous operation in case data reading has to be done at the start of operation, since such data reading is conducted during the initial unstable state of the power supply.
Control of a recording apparatus by the output of an oscillating circuit of a determined frequency is quite well known. Such controller method is however not precise enough due to fluctuation or time-dependent change of performance in the components constituting oscillating circuit, and is costly because of a large number of components involved in such oscillating circuit.
In a recording apparatus equipped with a mechanism for manual sheet insertion, the sheet feeding into the apparatus is effected at a determined timing after the detection of sheet insertion. Consequently, even if the sheet is pulled out immediately after it is inserted, the sheet feeding mechanism is activated to cause a wasteful operation of the recording apparatus.
In a recording apparatus, for example a copier in which the original document performs reciprocating motion, it is already known, for controlling the forward and backward motion of the original document, to activate a timer in response to the detection of the start of forward motion of an original carriage and to invert the moving direction upon expiration of a determined period measured by said timer. Such controlling method is however incapable of achieving exact inversion due to fluctuation in the performance of the timer or other components.
In a recording apparatus for information recording on a recording sheet, the feeding of the recording sheet is started from a determined position when a scanning unit for scanning the original document reaches a determined position, in order to record the image in a proper position on the recording sheet. However a slight error in sheet positioning is unavoidable if the recording sheets are to be fed to said determined position from plural supply sources, such as a sheet cassette and a manual sheet insertion unit.
In a recording apparatus in which an original document is scanned by a scanning unit, it is already well known to obtain a trigger signal or a timing signal required for controlling the apparatus in the course of movement of said scanning unit. Plural trigger or timing signals, if required, may be obtained from plural sensors provided for detecting the movement of said scanning unit. Such method however requires a large number of sensors according to the number of desired signals, thus inevitably leading to a complicated structure.
Recent control of recording apparatus with a microcomputer has enabled various functions of self-diagnosis and abnormality detection. However the increasing number of indicators required for indicating thus detected abnormalities not only complicates the inspecting work of the operator but also leads to a larger space for installation of such indicators and a complicated mechanism.
Temperature control, for example for the fixing device of a copier, has generally been achieved by terminating the waiting mode when said fixing device reaches a determined target temperature and performing on-off control of a heater. Such control method is however associated with a long waiting time because the possible temperature overshooting precludes the use of a high-power heater. In order to overcome such drawback, it is also proposed to employ a detecting point lower than the target control temperature in combination with a high-power heater and to terminate the waiting mode when the temperature reaches said detecting point, thereby reducing the waiting time. In such method, however, the timing for terminating the waiting mode is difficult to determine since the temperature control is effected independently from the initial temperature state of the fixing device. More specifically, the state of temperature rising of the fixing device varies according to the initial temperature thereof. Consequently, if the temperature rise is slow, the fixing device may not reach the target control temperature when the actual fixing operation is needed, after passing the aforementioned detecting point.
In order to detect the failure of a temperature control element such as a thermistor, it is already known to inspect the change in resistance after the lapse of a determined period and to identify a failure in case of the absence of change in resistance. Such control method may lead to overheating of the heater if the power supply is manually turned on and off repeatedly before the lapse of said determined period.
Also in a temperature control method utilizing the change in resistance of a temperature control element such as a thermistor, a precise temperature control in a particular range is difficult to achieve because of the temperature-resistance characteristic of such thermistor and of the narrow dynamic range of the analog-to-digital converter receiving the resistance-indicating signal from the thermistor. Besides said narrow dynamic range is the cause of inability for detecting an abnormally high temperature of the heater controlled by said thermistor.
Furthermore, there are already known various methods for detecting the failure of thermistor or halogen heater, but such methods cannot be utilized for detecting abnormalities including those in the driving circuits for such thermistor and halogen heater.
Conventional copier or laser beam printer is usually equipped with various indicators such as those for indicating the copy number and for indicating sheet jamming, and various operating means such as a copy switch and numeral keys, and requires a very complicated operations for an unskilled operator. Thus, in case the apparatus interrupts the recording operation in response to the detection of absence of recording sheet, the operator may misunderstand such interruption as the end of a series of recording operations and renew the setting of the copy number with the numeral keys.
Furthermore, in a conventional recording apparatus with a sheet feeding mechanism, such as a copier or a laser beam printer, the absence of recording sheet is indicated as soon as the sheets run out. However an unskilled operator may turn off the power supply immediately in such situation, and such premature turning off results in insufficient cleaning of the photosensitive drum thus leaving an unnecessary charge on the drum and causing unevenness in the reproduced image thereafter.